CITRIC ACID 23
any trace of calcium sulphate. Its crystals should not be deliquescent owing
to the presence of traces of sulphuric acid. It may give 0-05-0-25% of ash ;
it is always necessary to test for copper and lead, and to determine the latter.
The presence of these poisonous metals is accidental, and as a rule they do not
exceed 0-01% in commercial citric acid. The proportion of lead varies some-
what ; samples of English origin have shown from 0-0018 to 0-024%, French
and German samples from 0-0006 to 0-0029%, and American samples from 0-003
to 0-0063%. According to some authorities the maximum limit allowable
should be 0-002% of lead, whilst others give 0-5 m. grm. per 100 grams. Not
infrequently commercial citric acid is adulterated with tartaric acid.
Lemon Juice, etc.
The juice pressed from lemons contains citric acid and is used mainly
for the preparation of calcium citrate and thus of citric acid. The crude
juice (agro crudo] is a greenish-yellow liquid with an acid taste resembling
that of the lemon in the fresh juice ; later the taste becomes bitter. The
concentrated juice (agro cotto] is a dense, syrupy, brown liquid with an
odour recalling that of caramel and a bitter, highly acid flavour.
The bergamot and wild lemon (Citrus limetta] also give crude and con-
centrated juice, which differ somewhat in objective properties from that
of the lemon ; bergamot juice, prepared specially in Calabria, is also used
for making calcium citrate.
Analysis of the juice includes determinations of the specific gravity,
free acidity, citric acid and other organic acids united with bases, true
citric acid, alcohol and adulterants, these being usually free mineral acids
or salt water.
1. Specific Gravity.—This is measured with a hydrometer or a Mohr's
balance. Use is also made of a citvometer, which is a hydrometer on which
60 degrees corresponds with the specific gravity 1-24, this being a standard
for concentrated juice.
2. Free Acid.—50 c.c. of the concentrated juice are diluted to 500 c.c.
with water and 25 c.c. (=2-5 c.c. of juice), then titrated with N/2-soda,
using neutral litmus paper as indicator. With the non-concentrated juice,
10 or 20 c.c. are taken directly. In any case, before complete neutralisation,
when about five-sixths of the free acidity has been neutralised, the liquid
is boiled for a few minutes and the titration then concluded. The acidity
is calculated as crystallised citric acid; i c.c. N/2-alkali = 0-035 gram
of CCH807 + HaO. To give the result in ounces per gallon, after the
English way, the percentage found is multiplied by 1-60.
3. Citric and other Organic Acids combined with Bases.—The
neutral solution remaining from the preceding determination is evaporated
to dryness and the residue, after cautious incineration, treated with water
and with a measured volume of N-sulphuric acid ; after boiling and filtering,
the excess of sulphuric acid in the filtrate is determined by titration with
N-alkali. The amount of sulphuric acid used to neutralise the ash is equiva-
lent to the total organic acids in the substance, since all the organic salts
are transformed into carbonates on incineration. Hence, if the total acids
are calculated as citric acid (i c.c. of N-sulphuric acid = 0-070 gram of
crystalline citric acid, C0H807 + H20) and from this is deducted the freehave very little hardness